October 2006

ll species of spider—about 40,000 at last count—extrude silk from modified limbs, called spinnerets, on their abdomens. Many species, including the orb weavers, produce a whopping seven different kinds of silk. Two are gooey, but the other five silks are fibrous and together create an armamentarium that enables the orb weavers to perform weight-defying rope tricks. For twenty years workers have been trying to reproduce the showiest of the fibrous silks, the so-called dragline silk, which serves the spiders as rappel lines (for dropping in like Spiderman) or as radial trusses for a web.

In laboratory testing, dragline silk rivals such artificial polymers as nylon and Kevlar (the fiber of bulletproof vests), in stiffness and strength. Recently, though, two biomechanists showed that the four other fibrous silks are also worth imitating—perhaps even more so. Learning how to spin all five of those silken wares would be a boon not only to the building-minded, but also to the planet. After all, silk is synthesized in a spider’s belly, an ecofriendly environment, solely out of biodegradable ingredients. It’s even edible, at least to an arachnid: spiders eat their old webs to get extra protein. Looking at spider silks with such a panoply of properties could give valuable insight into a manufacturing process and a set of ingredients that might be adapted for the making of new, high-performance fibers.

Todd A. Blackledge, a behavioral ecologist at the University of Akron in Ohio, and Cheryl Y. Hayashi, an evolutionary biologist at the University of California, Riverside, managed to get all five kinds of fibrous silk for their experiments from the silver garden spider (Argiope argentata) [see illustrations below]. In addition to dragline, they gathered the silk that functions as a temporary scaffold, holding the web together until a spiral of “capture” silk is laid down. Third, the biomechanists confiscated and unraveled spider-egg sacs to get a grooved fiber that protects the developing spiderlings from thumps and bumps. Fourth was the silk from the aciniform gland, used for wrapping and restraining prey. Finally, they collected the sticky and stretchy capture silk that forms the more permanent, spiral interior of the web: the stuff that does the morbid work of netting insect prey.

Five kinds of silk spun by an orb weaver spider differ in breaking strength and elasticity: (1) Dragline silk forms the radial spokes of the web; (2) capture silk, the sticky silk that captures prey, stretches far more than any other silk; (3) egg-sac silk protects the spider’s developing offspring; (4) prey-wrapping silk—recently shown to be the toughest silk of all—binds prey; (5) temporary silk forms a spiral scaffold for the web and is ultimately replaced with stronger and more elastic silks.

The investigators tested the five silks by stretching short sections of the individual fibers on a new kind of testing frame. Silk fibers are strong, but they’re also so wispy that testing them takes a device of exceptional sensitivity and accuracy; until now, no one had been able to test temporary, egg-sac, or prey-wrapping silk from the same species of spider. (Both dragline and capture silk from a number of different kinds of spiders had been measured for stiffness, strength, and toughness.) The new testing frame can measure forces as small as a nanonewton—roughly the weight of the dried ink in the period at the end of this sentence.

To a materials scientist, stiffness is a measure of how much a sample stretches or deforms (as a percentage of its length) when a force is applied. Dragline silk, for instance, stretches just about as much as would a nylon thread of comparable diameter, if a spider fell on it. In casual conversation, strength sometimes means much the same thing. In its technical sense, though, “strength” is breaking strength: how much weight a strand can bear before it breaks. Here, too, spider silk is superlative; the breaking strength of dragline silk is about that of most steels.

By the third measure, toughness, capture silk was thought to be the standout. Toughness is a kind of combination of stiffness and strength, a measure of how much energy can be absorbed (by stretching and bearing weight) before the material fails. In toughness, both capture silk and dragline silk trump Kevlar.

When they tested the spider’s full toolkit for the first time, Blackledge and Hayashi confirmed the amazing properties of dragline and capture silk. But they also discovered that some of the highest-performance silks had been overlooked. The other three fibrous silks are all stiffer than dragline, at least for small loads, and egg-sac silk stretches to more than 50 percent of its resting length, nearly twice as much as dragline, before it breaks. The sticky silk of the capture spiral is still the flexibility champion, and dragline reigns as the strongest silk; but temporary silk and prey-wrapping silk, both stiffer than dragline, do nearly as well in strength [see graphs below].

The “stiffness,” or relation between stretch and tensile force applied, is plotted on the graph; the upper end of each curve represents the breaking strength of the material.

The big surprise is that prey-wrapping silk is a real Kevlar killer. It can absorb twice the energy of any other silk before it breaks. Presumably the intimate, long-term contact with an insect struggling for its life demands a tougher material than do the single impacts that dragline must sustain. If prey-wrapping silk can be imitated, tougher, lighter-weight bulletproof fabrics should be in the offing.

With recent advances in understanding the biochemical content of silk and the genetics of the proteins that make it up, there is real promise that artificial spider silk can be made. Understanding the material properties of the various kinds of spider silk might help fine-tune and vary the basic manufacturing process for nonsuperman-made fibers. Personally, I’d like little nozzles that shoot the various kinds of silk from my wrists. I wouldn’t just swing around; I’d fight crime. I promise.

Adam Summers (asummers@uci.edu) is an assistant professor of bioengineering and of ecology and evolutionary biology at the University of California, Irvine.

Copyright © Natural History Magazine, Inc., 2006